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Wed, 14 Aug 2019 07:54:18 -0700 (PDT) Subject: Re: [PATCH v2] Add flags option to get xattr method paired to __vfs_getxattr To: Jan Kara References: <20190813145527.26289-1-salyzyn@android.com> <20190814110022.GB26273@quack2.suse.cz> From: Mark Salyzyn Message-ID: <71d66fd1-cc94-fd0c-dfa7-115ba8a6b95a@android.com> Date: Wed, 14 Aug 2019 07:54:16 -0700 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:60.0) Gecko/20100101 Thunderbird/60.8.0 MIME-Version: 1.0 In-Reply-To: <20190814110022.GB26273@quack2.suse.cz> Content-Language: en-GB X-CRM114-Version: 20100106-BlameMichelson ( TRE 0.8.0 (BSD) ) MR-646709E3 X-CRM114-CacheID: sfid-20190814_075420_669855_BD4C6E3F X-CRM114-Status: GOOD ( 22.08 ) X-BeenThere: linux-mtd@lists.infradead.org X-Mailman-Version: 2.1.29 Precedence: list List-Id: Linux MTD discussion mailing list List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Cc: Latchesar Ionkov , Dave Kleikamp , jfs-discussion@lists.sourceforge.net, Phillip Lougher , linux-integrity@vger.kernel.org, Martin Brandenburg , samba-technical@lists.samba.org, Dominique Martinet , Chao Yu , Mimi Zohar , Adrian Hunter , linux-mm@kvack.org, Chris Mason , netdev@vger.kernel.org, Andreas Dilger , linux-xfs@vger.kernel.org, Eric Paris , linux-f2fs-devel@lists.sourceforge.net, linux-afs@lists.infradead.org, Stephen Smalley , Mike Marshall , devel@driverdev.osuosl.org, linux-cifs@vger.kernel.org, Paul Moore , Sage Weil , "Darrick J. Wong" , Richard Weinberger , Mark Fasheh , linux-unionfs@vger.kernel.org, Hugh Dickins , James Morris , cluster-devel@redhat.com, Joseph Qi , Vyacheslav Dubeyko , Casey Schaufler , v9fs-developer@lists.sourceforge.net, Ilya Dryomov , linux-ext4@vger.kernel.org, kernel-team@android.com, devel@lists.orangefs.org, Serge Hallyn , Gao Xiang , Eric Van Hensbergen , ecryptfs@vger.kernel.org, linux-erofs@lists.ozlabs.org, Josef Bacik , reiserfs-devel@vger.kernel.org, Bob Peterson , Joel Becker , Anna Schumaker , David Sterba , Jaegeuk Kim , ceph-devel@vger.kernel.org, selinux@vger.kernel.org, Trond Myklebust , Andreas Gruenbacher , David Howells , linux-nfs@vger.kernel.org, Theodore Ts'o , linux-fsdevel@vger.kernel.org, Artem Bityutskiy , Mathieu Malaterre , Greg Kroah-Hartman , Miklos Szeredi , Jeff Layton , linux-kernel@vger.kernel.org, stable@vger.kernel.org, Tyler Hicks , Steve French , =?UTF-8?Q?Ernesto_A=2e_Fern=c3=a1ndez?= , linux-btrfs@vger.kernel.org, linux-security-module@vger.kernel.org, Jan Kara , Tejun Heo , linux-mtd@lists.infradead.org, Andrew Morton , David Woodhouse , "David S. Miller" , ocfs2-devel@oss.oracle.com, Alexander Viro Content-Transfer-Encoding: 7bit Content-Type: text/plain; charset="us-ascii"; Format="flowed" Sender: "linux-mtd" Errors-To: linux-mtd-bounces+linux-mtd=archiver.kernel.org@lists.infradead.org On 8/14/19 4:00 AM, Jan Kara wrote: > On Tue 13-08-19 07:55:06, Mark Salyzyn wrote: > ... >> diff --git a/fs/xattr.c b/fs/xattr.c >> index 90dd78f0eb27..71f887518d6f 100644 >> --- a/fs/xattr.c >> +++ b/fs/xattr.c > ... >> ssize_t >> __vfs_getxattr(struct dentry *dentry, struct inode *inode, const char *name, >> - void *value, size_t size) >> + void *value, size_t size, int flags) >> { >> const struct xattr_handler *handler; >> - >> - handler = xattr_resolve_name(inode, &name); >> - if (IS_ERR(handler)) >> - return PTR_ERR(handler); >> - if (!handler->get) >> - return -EOPNOTSUPP; >> - return handler->get(handler, dentry, inode, name, value, size); >> -} >> -EXPORT_SYMBOL(__vfs_getxattr); >> - >> -ssize_t >> -vfs_getxattr(struct dentry *dentry, const char *name, void *value, size_t size) >> -{ >> - struct inode *inode = dentry->d_inode; >> int error; >> >> + if (flags & XATTR_NOSECURITY) >> + goto nolsm; > Hum, is it OK for XATTR_NOSECURITY to skip even the xattr_permission() > check? I understand that for reads of security xattrs it actually does not > matter in practice but conceptually that seems wrong to me as > XATTR_NOSECURITY is supposed to skip just security-module checks to avoid > recursion AFAIU. Good catch I think. I was attempting to make this change purely inert, no change in functionality, only a change in API. Adding a call to xattr_permission would incur a change in overall functionality, as it would introduce into the current and original __vfs_getxattr a call to xattr_permission that was not there before. (I will have to defer the real answer and requirements to the security folks) AFAIK you are correct, and to make the call would reduce the attack surface, trading a very small amount of CPU utilization, for a much larger amount of trust. Given the long history of this patch set (for overlayfs) and the large amount of stakeholders, I would _prefer_ to submit a followup independent functionality/security change to _vfs_get_xattr _after_ this makes it in. > >> diff --git a/include/uapi/linux/xattr.h b/include/uapi/linux/xattr.h >> index c1395b5bd432..1216d777d210 100644 >> --- a/include/uapi/linux/xattr.h >> +++ b/include/uapi/linux/xattr.h >> @@ -17,8 +17,9 @@ >> #if __UAPI_DEF_XATTR >> #define __USE_KERNEL_XATTR_DEFS >> >> -#define XATTR_CREATE 0x1 /* set value, fail if attr already exists */ >> -#define XATTR_REPLACE 0x2 /* set value, fail if attr does not exist */ >> +#define XATTR_CREATE 0x1 /* set value, fail if attr already exists */ >> +#define XATTR_REPLACE 0x2 /* set value, fail if attr does not exist */ >> +#define XATTR_NOSECURITY 0x4 /* get value, do not involve security check */ >> #endif > It seems confusing to export XATTR_NOSECURITY definition to userspace when > that is kernel-internal flag. I'd just define it in include/linux/xattr.h > somewhere from the top of flags space (like 0x40000000). > > Otherwise the patch looks OK to me (cannot really comment on the security > module aspect of this whole thing though). Good point. However, we do need to keep these flags together to reduce maintenance risk, I personally abhor two locations for flags bits even if one comes from the opposite bit-side; collisions are undetectable at build time. Although I have not gone through the entire thought experiment, I am expecting that fuse could possibly benefit from this flag (if exposed) since it also has a security recursion. That said, fuse is probably the example of a gaping wide attack surface if user space had access to it ... your xattr_permissions call addition requested above would be realistically, not just pedantically, required! I have to respin the patch because of yet another hole in filesystem coverage (I blew the mechanical ubifs adjustment, adjusted the wrong function), so please do tell if my rationalizations above hit a note, or if I _need_ to make the changes in this patch (change it to a series?). Thanks -- Mark Salyzyn ______________________________________________________ Linux MTD discussion mailing list http://lists.infradead.org/mailman/listinfo/linux-mtd/